1. Field of Invention
The subject invention is generally related to systems for delivering drilling fluids (mud) for oil and gas drilling applications and is specifically directed to a method and apparatus for varying the density of mud in deep water oil and gas drilling applications.
2. Description of the Prior Art
It is well known to use drilling mud to drive drill bits, maintain hydrostatic pressure and to carry away particulate matter when drilling for oil and gas in subterranean wells. Basically, the drilling mud is pumped down the drill pipe and provides the fluid driving force for the drill bits, and then it flows back up from the bit along the periphery of the drill pipe and inside the open hole and casing for removing the particles loosed by the drill bit. At surface the returning mud is cleaned to remove the particles and recycled down into the hole.
The density of the drilling mud is monitored and controlled in order to maximize the efficiency of the drilling operation and to maintain the hydrostatic pressure. In a typical application, a well is drilled using a drill bit mounted on the end of a drill stem inserted down the drill pipe. The mud is pumped down the drill pipe and through the drill bit to drive the bit. A gas flow is also pumped and/or other additives are also pumped into the drill pipe to control the density of the mud. The mud passes through the drill bit and flows upwardly along the drill string inside the open hole and casing, carrying the loosed particles to the surface.
One example of such a system is shown and described in U.S. Pat. No. 5,873,420, entitled: xe2x80x9cAir and Mud Control System for Underbalanced Drillingxe2x80x9d, issued on Feb. 23, 1999 to Marvin Gearhart. The system shown and describe in the Gearhart patent provides for a gas flow in the tubing for mixing the gas with the mud in a desired ration so tat the mud density is reduced to permit enhanced drilling rates by maintaining the well in an underbalanced condition.
It is known that there is a preexistent pressure on the formations of the earth, which, in general, increases as a function of depth due to the weight of the overburden on particular strata. This weight increases with depth so the prevailing or quiescent bottom hole pressure is increased in a generally linear curve with respect to depth. As the well depth is doubled, the pressure is likewise doubled. When drilling in deep water or ultra deep water this is further complicated because of the pressure on the sea floor by the water above it. Thus, high-pressure conditions exist at the beginning of the hole and increase as the well is drilled. It is important to maintain a balance between the mud density and pressure and the hole pressure. Otherwise, the pressure in the hole will force material back into the well bore and cause what is commonly known as a blowout. In basic terms, the gases in the well bore flow out of the formation into the well bore and bubble upward. When the standing column of drilling fluid is equal to or greater than the pressure at the depth of the borehole the conditions leading to a blowout are minimized. When the mud density is insufficient, the gases or fluids in the borehole can cause the mud to decrease in density and become so light that a blowout occurs.
Blowouts are a threat to drilling operations and a significant risk to both personnel and the environment. Typically blowout preventers or BOP""s are installed at the ocean floor to minimize a blowout from an out-of-balance well. However, the primary method for minimizing blowout is the proper balancing of the drilling mud density to maintain the well in balance at all times. While BOP""s can contain a blowout and minimize the damage to personnel and the environment, the well is usually lost once a blowout occurs, even if contained. It is far more efficient and desirable to use proper mud control techniques in order to reduce the risk of a blowout than it is to contain a blowout once it occurs.
In order to maintain a safe margin, the column of drilling mud in the annular space around the drill stem is of sufficient weight and density to produce a high enough pressure to limit risk to near zero in normal drilling conditions. While this is desirable it also slows the drilling process. In some cases underbalanced drilling has been attempted in order to increase the drilling rate. However, to the present day the mud density is the main component for maintaining a pressurized well under control.
Deep water and ultra deep water drilling has its own set of problems coupled to the need to provide a high density mud in a well bore that starts several thousand feet below sea level. The pressure at the beginning of the hole is equal to the hydrostatic pressure of the seawater above it, but the mud must travel from the sea surface to the sea floor before its density is useful. It is well recognized that it would be desirable to maintain mud density at or near seawater density (or 8.6 PPG) when above the borehole and at a heavier density from the seabed down into the well. In the past pumps have been employed near the seabed for pumping out the returning mud and cutting from the seabed above the BOP""s and to the surface using a return line that is separate from the riser. This system is expensive to install, requiring separate lines, expensive to maintain and very expensive to run. Another experimental method employs the injection of low-density particles such as glass beads into the returning fluid in the riser above the sea floor to reduce the density of the returning mud as it is brought to the surface. Typically, the BOP stack is on the sea floor and the glass beads are injected above the BOP stack.
While it has been proven desirable to reduce the mud density above the sea floor there are not any prior art techniques that effectively accomplish this objective.
The subject invention is directed a method and apparatus for controlling drilling mud density above the sea floor, and when the BOP stack is on the seabed above the stack, of wells in deep water and ultra deep water applications. It is an important aspect of the invention that the mud is diluted using base fluid. The base fluid is of lesser density than the mud required at the wellhead and by combining the two a diluted mud results. In the preferred embodiment of the invention, the base fluid has a density less than seawater (or less than 8.6 PPG). By combining the appropriate quantities of drilling mud with base fluid, a riser mud density at or near the density of seawater may be achieved. It is an important feature of the invention that no additional hardware is required below the surface. The riser charging lines are used to inject the low-density base fluid at or near the BOP stack on the seabed. The cuttings are brought to the surface with the diluted mud and separated in the usual manner. The diluted mud is then passed through a centrifuge system to separate the heavier drilling mud from the lighter base fluid.
In the example where the desired riser mud density is 8.6 PPG, or that of seawater, it can be assumed the base fluid is an oil base having a density of approximately 6.5 PPG. Using an oil base mud system, for example, the mud may be pumped from the surface through the drill string and into the bottom of the well bore at a density of 12.5 PPG, typically at a rate of around 800 gallons per minute. The fluid in the riser, which is at this same density, is then diluted above the sea floor with an equal amount or more of base fluid through the riser charging lines. The base fluid is pumped at a faster rate, say 1500 gallons per minute, providing a return fluid with a density that can be calculated as follows:
(FMixc3x97Mi)+(FMbxc3x97Mb)/(FMi+FMb)==Mr,
where:
FMi=flow rate Fi of fluid,
FMb=flow rate Fb of base fluid into riser charging lines,
Mi=mud density into well,
Mb=mud density into riser charging lines, and
Mr=mud density of return flow above the sea floor in riser.
In the above example:
Mi=12.5 PPG,
Mb=6.5 PPG,
FMi=800 gpm, and
FMb=1500 gpm.
Thus, the density Mr of the return mud can be calculated as:
Mr=((800xc3x9712.5)+(1500xc3x976.5))/(800+1500)=8.6 PPG.
The flow rate, Fr, of the mud having the density Mr in the riser is the combined flow rate of the two flows, Fi and Fb. In the example, this is:
Fr=Fi+Fb=800 gpm+1500 gpm=2300 gpm.
The return flow in the riser above the BOP""s is a mud having a density of 8.6 PPG (or the same as seawater) flowing at 2300 gpm. This mud is returned to the surface and the cuttings are separated in the usual manner. Centrifuges at the surface will then be employed to separate the heavy mud, density Mi, from the light mud, density Mb.
The system of the subject invention is particularly useful because it can be retrofitted on existing offshore rigs without requiring any additional hardware below the surface. The conduits and centrifuges required are all placed at the surface. The riser charging lines are employed to deliver the base fluid.
It is, therefore, an object and feature of the subject invention to provide a new and useful method and apparatus for diluting the mud density in the riser of a deep water or ultra deep water well.
It is another object and feature of the subject invention to provide a method and an apparatus for diluting mud density in a riser without adding any additional hardware to a riser system beneath the surface of a deep water or ultra deep water drilling installation.
It is an additional object and feature of the subject invention to provide a method and apparatus for diluting mud density in deep water and ultra deep water drilling applications for both drilling units and floating platform configurations. It is yet another object and feature of the subject invention to provide a method for diluting the density of mud in a riser by injecting low density fluids into the riser charging lines or riser systems with surface BOP""s.
It is a further object and feature of the subject invention to provide an apparatus for separating the low density and high-density fluids from one another at the surface.
Other objects and features of the invention will be readily apparent from the accompanying drawing and detailed description of the preferred embodiment.